Philips Stirling Engine

FtO. 12.4 Type 102C SlirliiiR-cnpincelecirie-powcr Rcneralingset (after Mcijei 1969c)

miles) al a speed of 4.6 m/s (9 knots) on the Dutch canals. Viewers on shore were apparently startled at the sight of the boat with several passengers moving without noise through the water with no visible means of propulsion. The engine was concealed under a cardboard box.

The decision was made in 1952 to start production of the 102C generator sets. Initial production was set at five series of 50 engines. In the end about 100 were made before production was stopped in 1954 and further effort was concentrated on larger engines. The reason was not so much the fault of the engine as the lack of a market.

The original application lor radio sets had largely disappeared with the invention of the transistor, the development of electric storage batteries, and improvements in radio valves. No substitute market could be found or foreseen and finally, about 1958, the Type 102C generators were dispersed to universities and technical colleges in Europe.

An unexpected application of the unit was made in February 1953 as part of the relief work at the time of the great sea floods in Holland. A contemporary newspaper report filed from the inundated village of s'Gravcndeel describes how seventeen of the generators from the Philips factory at Dordrecht were used for emergency lighting in hospitals, and refugee centres and for amateur radio installations providing communications for the Red Cross. One flat-bottomed boat with two hot-air engines for propulsion carried blankets and foodstuff for a thousand people!

The engine of the Type 102C generator perhaps represented the acme of air engine technology, for subsequently all development was concentrated on larger engines using helium or hydrogen as the working fluid. The performance characteristics of the Type 102C engine were never published by Philips nor, surprisingly, by any of the recipients of the engine alter their final dispersal. It was left to Ward (1972) to conduct careful measurements of the engine modified to operate with water cooling and on liquid petroleum gas. The operating characteristics were finally published by Walker et eil. (1978) and arc reproduced here in Chapter 9. The U.S. Navy developed an inquisitive interest in the small engines developed by Philips. Several engines were acquired and tested with results reported by Schräder (1951).

Many other varieties of engines were also considered by Philips. Fig. 12.5 shows a configuration of opposed piston engine that was investigated experimentally.

Little is known of the I 1 kW (15 hp) double-acting engine. A contemporary newspaper report contains an account and photograph of a demonstration of the engine to Henry Ford II in 1948. Percival (1974) recounts how the attempt in 1950 by General Motors to form a working agreement with Philips was rebuffed as premature.

Meijer (1959a) identifies the reasons foi the failure to persist with the development of the double-acting engine as 'an exceptionally intractable lubrication problem', thought to be the dry-rubbing piston seals in each cylinder separating the hot and cold spaces. The similar problem in displacer engines was eventually overcome with the development of Rulon seals and this permitted the resumption of work on double-acting engines. Another less significant factor was the aerodynamic and ther-modyamic penalties of this type of engine. The volume variations in the hot and cold spaces could no longer be chosen freely as regards magnitude and phase.

cryogenic phase

Köhler (1965) has recounted how one of the small Philips air engines was found to operate effectively as a refrigerator, With hydrogen as the working fluid the cylinder head (expansion space) cooled to such a low

Expansion piston

Ilcatei / y— Regenerator Coolct

Compressinn spacc

-Compression piston

Expansion piston

-Compression piston

Pic:. 12.5. Configuration of opposed-piston engine.

Pic:. 12.5. Configuration of opposed-piston engine.

temperature that atmospheric air condensed upon it. This led to the formation about 1948 of a research group, headed by Köhler, to develop the cryogenic cooler. This team was brilliantly successful and the first cryogenic cooling engines were being sold in 1954. These machines produced liquid air at the rate of about 2.22 cmVs (8 litres per hour) Köhler and Jonkers (1954a and b) have provided a full account of the fundamentals and construction of the gas refrigerating engine.

This machine and other later developments were manufactured in large numbers and have been sold all over the world. It may be considered the outstanding commercial success for modern Stirling engines.

Further details of the Philips work on refrigerating machines and associated cryogenic equipment arc not included here because of limited space, but there arc many important contributions from Philips in the cryogenic literature. The material is fully covered in a forthcoming companion work to this volume entitled Regenerative Cooling Engines.

RHOMI1IC J'ltASP

A fresh start

The rhombic drive for Stirling engines was invented by Rolf Meijer in 1953. Its adoption in 1954 for all engines represented virtually a fresh start on Stirling engines for Philips.

A first account of tlie new engine configuration was given by Mcijcr (1959a)—some twelve years after the final paper by van Wccnan about the early air engines. Mcijcr described the single-cylinder rhombic-drive engine of 30 kW (40 hp: the Philips Type 1-365) shown in Fig. 12.6. This engine had a cylinder bore of 88 mm (3.46 in), a piston stroke of 6tl mm (2.36 in), a speed of 1500 revolutions per minute and it used hydrogen as the working fluid at nominal heater and cooler temperatures of 700 "C (1290 °F) and 15°C (60 °F). The mean pressure was 10.3 MN/m2, (1500 lb per sq in) the maximum pressure was 13.7 MN/m* (2000 lb per sq in) and the compression ratio pIMni/pinill was two. Various operating characteristics of the engine, reproduced in Chapter 9, were given for output and efficiency as a function of the pressure of the working fluid and the engine speed. I he effect of heater and cooler temperatures on engine power and thermal efficiency at constant speed and mean pressure were also given. The paper included a comparison of the fuel consumption contour chart, in terms of mean effective pressure and speed for the new Stirling engine and a commercial diosel engine.

Philips Stirling Engine Cooler

Annular connecting duel

-Fxhaust outlet

-Fuel atomizer

Prcliealer — Ilurncr

Heritor tubes — Fins

-Expansion space --Regenerator

— Cooler tubes — Displaccrdome

-Cylinder wall

-Displacer

Piston Displacer rod seal

Displacer rod Piston rod seal Piston rod - Piston yoke

Counterweights Displacer yoke

Hut uer-air inlet-Compression space—-Ruffe i spacc

Piston connccting rod

Displacer connecting roil

Fig. 12.6. Crass-section of rhombic-drive engine (after Mcijcr 1969c).

Rhombic drive

The same data, but in considerably more detail, was given by Meijer (1960a) in a reprint of his doctoral thesis. This is a most valuable document, being a comprehensive mathematical analysis of the dynamics of the rhombic drive. Some features arc discussed in Chapter 6. A similar kinematic arrangement for reciprocating engines was invented by the renowned British automotive engineer, Lanchester in 1893 (Crabtree 1976). The Lanchester drive for horizontal llat twin internal-combustion engines is so remarkably similar to the rhombic drive, that it must be considered a blood relation even if not truly the grandfather of the rhombic drive. All the essential features are there, just as the Siemens engine predated the double acting engine by 80 years.

Tubular heater

The new rhombic-drive engine was dramatically different to the earlier small air engine. A cross-section of the rhombic-drive engine is shown in Fig. 12.6. The heater, also shown in Fig. 12.7, consisted of a complicated tubular structure with a horizontal annular manifold at the lop. Alternate heater tubes were connected to the cylinder head and to the regenerator housings with the tubes to the regenerator housings provided with fins for improved heal transfer.

Split regenerator

The regenerator on the new engine was divided into ten units contained in small cups instead of being a single annulus around the cylinder. This overcame the problem of thermal stress distribution of earlier designs. Three heater tubes were connected to each regenerator cup.

Water cooler

Water cooling was used on the new engine with the working fluid passing through a tubular cooler mounted below the regenerator cups. The design of the cooler was adapted to that of the regenerator so that a number of tubes were bunched together in groups, the ends of each cooler unit protruding into a regenerator cup. The tubes were mounted with a sliding fit in the cooler housing so as to allow unrestrained axial movement of the regenerator cups due to thermal movement of the heater tubes.

Air preheater

The heater was surrounded by the exhaust-gas/air preheater shown in Fig. 12.8. This was a recuperative type heater with spirally curved plates forming narrow passages through which exhaust gas and fresh air passed in alternate channels.

l io. 12.7. Heater assembly for rhombic-drive engine (afler Mcijcr 1969c).

I lie burner arrangement is shown in Fig. 12.8. The single burner was centrally located within the ring of heater tubes and was of the 'swirl-chamber' type suitable for use with a variety of gaseous and liquid fuels. An atomizer was used for operation with liquid fuels.

Power control for the engine was obtained by a hydraulic governor. This adjusted the pressure of the working fluid in the engine to maintain a prc-selected speed of operation. A thermostat control was included to regulate the fuel supply to maintain the heater tubes at a constant maximum temperature. These controls are described in more detail in Chapter 10.

Gas seals

To save weight, the crankcasc of the rhombic-drive engine was not pressurized and so the question of sealing the reciprocating piston and displacer was critical. Seals were required against the egress of working

fuel iruóctor)

(Thermocouple)

I leatcr manifold

(Air prehcalcr)

21 Heater tube

IS<Enginc cylinder)

Regenerator

Inlet air stream Exhaust product stream

Pick 12.8. Cross-section of hot-parts assembly of the rhombic-drive engine, (a) vertical section, (b) horizontal section.

fluid (hydrogen) and also the ingress to the working space of crankcasc lubricant. Seals are nol mentioned in either of the papers by Mcijer given in 1959 and 1961 but some details of Philips contemporary seal practice have been given by Percival (1974).

On the early air engines it was found that carbon piston-rings similar to those used in air compressors resulted in unacceptable levels of contamination of the heat exchangers, and probably blockage of the fine interstices of the regenerator. Therefore, on the displacer of the air engines, a close clearance wear band was used. For the power piston a combination of three to five conventional iron piston rings was found to be adequate for reasonable life but it was dillicult to prevent the ingress of oil to the working space with consequent contamination and blockage of the regenerator.

Development of the rhombic drive eliminated the side loading of the piston against the cylinder wall that is characteristic of conventional crank-slider mechanisms. Because of this, piston rings were not required on the piston for sealing and guiding. Instead from about 1955 to 1960 Philips developed close tolerance seals for both the power piston and the displacer. Percival (1974) describes these as a tin-lead alloy band with circumferential grooves and treated with molybdenum disulphide. The piston band was machined slightly oversize then shrunk in a dry ice bath for initial fitting. The piston was then honed into the cylinder by motoring the crank piston assembly for several hours. Pcrcival remarks that the procedure proved to be more of an art than a science and usually had to be repeated if the piston was removed for any reason. At best the seals were effective against leakage and had a slightly higher mechanical efficiency than piston rings.

Rolling seals

Clearly such a procedure was unsuitable for anything other than special applications. An apparent solution was at hand with the invention ol the rolling seal described by Rictdijk et al. (1965). Work on rolling seals, described also as 'positive seals' started at Philips in 1960. The result is shown in Fig. 12.9. It consists basically of a thin membrane of flexible material held by clamping rings against the stationary part and against the moving piston or rod. A small pressure difference across the seal was sufficient to hold it snug against the piston or cylinder wall so that it rolls olf these surfaces without creasing. Fig. 12.10 is a photograph of a rolling diaphragm.

By this time the working lluid pressures were as high as 14 MN/m ' (2000 lb per sq in) of hydrogen. It was simply not possible to contain this pressure across a diaphragm that was sulliciently thin and flexible to acfas a rolling seal. Therefore the system shown in Fig. 12.1 I was adopted. The flexible diaphragm was supported on a cushion of oil and the pressure of the oil was varied in sympathy with the gas pressure variation in the cylinder. A small pressure difference of less than 0.5 MN/nr (75 lb pei sq in) was maintained across the diaphragm and a high pressure difference was taken by an oil seal of conventional type separating the oil-filled space beneath the diaphragm from the crankcase. This was a much less demanding seal problem. To maintain the oil space at constant volume as the piston moved in the cylinder, it was necessary to 'step' the

Fj<3, 12.'.». Rolling seul piston and cylinder such thai:

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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